documentation/iommu: Remove description for qcom,smmu-v2

Remove qcom,smmu-v2 from the list of valid strings for
SMMU compatible property since the support for qsmmuv2
in the arm-smmu driver is no longer available.

Change-Id: Ice25da22d0dfb865ca02e98d639087b9b3086bbb
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>